In the case of fill level measurement according to the travel-time principle, signals, for instance, microwave signals or ultrasonic signals, are periodically sent toward the surface of a fill substance by means of a sending and receiving element and their echo signals reflected on the surface are then received following a distance dependent travel-time. An echo function representing echo amplitudes as a function of travel-time is formed. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the sending and receiving element.
From the echo function, a wanted echo is ascertained which probably corresponds to the reflection of the transmission signal on the fill substance surface. In such case, it is, as a rule, assumed that the wanted echo has the greatest amplitude, as compared with the remaining echoes. From the travel-time of the wanted echo, in the case of known propagation velocity of the transmission signals, the distance between the fill substance surface and the sending and receiving element, and, consequently, the fill level, is directly determined.
For determining the fill level, all known methods can be used which make possible the measurement of relatively short distances, e.g. under one hundred meters, by means of reflections of the transmission signals.
A known method is the frequency modulated continuous wave method (FMCW method) used in connection with fill level measuring devices working with microwaves. In FMCW methods, a microwave signal is continuously transmitted which is periodically frequency modulated, for example, on the basis of a sawtooth function. The frequency of the received echo signals exhibits, therefore, compared with the instantaneous frequency possessed by the transmission signal, at the point in time of its receipt, a frequency difference which depends on the travel-time of the echo signal. The frequency difference between the transmission signal and the received signal, which can be determined by a mixing of both signals and evaluating the Fourier spectrum of the mixed signal, corresponds then to the distance of the reflecting surface from the antenna. Additionally, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation corresponds to the echo amplitudes. This Fourier spectrum thus represents in this case the echo function.
Another known method is the pulse travel-time method which is used both in the case of fill level measuring devices working with microwaves and with fill level measuring devices working with ultrasonic waves. In the case of pulse travel-time methods, short, transmission signals, so called transmitted pulses, are periodically sent, which are reflected by the fill substance surface, and their echo signals are later received, following a distance-dependent travel-time. The received signal amplitudes as a function of time provides the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the sending and receiving element.
In the technology of fill level measurement, in such case, frequently a considerable effort is expended toward being able to execute reliable measurements even under difficult measuring conditions, e.g. in the case of fixedly installed disturbances in the container, stirrers sporadically protruding into the signal path or poor signal quality.
To this end, in part, very complex signal registering, signal conditioning, and/or signal evaluation methods are applied.
In a large number of applications, it is necessary additionally, for fill level measurements to monitor an exceeding or a falling beneath of one or more fixedly predetermined fill levels. Such a fixedly predetermined fill level is, for example, a fill level upper limit which must not be exceeded, in order to prevent an overfilling of the container. A further example is a fill level lower limit which must not be fallen beneath, e.g. in order to exclude the possibility of running pumps in a dry state.
The monitoring of predetermined fill levels thus serves for operational safety and is even, in some cases, required by law. For instance, the Water Management Law applicable in Germany contains such specifications.
Due to the safety relevance of the monitoring of predetermined fill levels, it is imperative that the monitoring run durably without error. Coupled therewith, the monitoring must satisfy high safety standards. Preferably, the functioning of the monitoring can be initially checked with reference to all measuring situations possibly arriving during operation.
In many applications, therefore, in addition to continuously operating fill level measuring devices, fill level limit switches are installed which monitor the exceeding, or falling beneath, of predetermined fill levels.
Signal registering, signal conditioning, and/or signal evaluation methods of conventional commercial fill level limit switches are, as a rule, markedly simpler in construction than fill level measuring devices. Correspondingly, it is easier to test for their error-free functioning, and they can be checked in advance for their ability to handle all measuring situations possibly arising during operation.
It represents, however, a considerable cost, space and maintenance requirement, to install these devices in addition to the fill level measuring devices.
It is possible to monitor exceeding or falling beneath the fixedly predetermined fill levels on the basis of fill levels measured with the continuously working fill level measuring device. However, since in the case of the described conventional fill level measuring devices, as a rule, complex signal registering, signal conditioning and/or signal evaluation methods are used, it is frequently not possible to test in advance the limit level monitoring performable with it, with reference to all the possibly arising measuring situations, for the purpose of eliminating possible erroneous measurements with certainty.